Yes you can copy the PO to the bill, button at the top right on the PO

A bill is used to record the vendor amount you need to pay, usually ONLY if you will not pay it immediately.  bills are a type of suspense file basically.  But you can if you wish enter the bill and pay it right away, it just takes more clicks

No bills are not required for the accounting, you can enter a check or credit card charge for the purchase directly if you wish.  But if you use enter bills, be sure you use pay bills to pay it - that is the best way to link the payment to bill and close it.